Noxious thermal stimuli excite specific somatic afferent fibers (A- delta and C) known to be essential for pain. The central nervous system (CNS) mechanisms and pathways for pain are not known, but previous research and/or clinical evidence suggests two major central systems are involved: (1) direct spinothalamic projections to posterior lateral (PO) and medial-intralaminar (M-IL) thalamus and (2) reticulo-thalamic projections from nucleus gigantocellularis of medulla (NGC). Accordingly, the PO and M-IL thalamus and NGC will be sampled, in the anesthetized squirrel monkey, for neurons responding exclusively or differentially to noxious heat. Antidromic and orthodromic CNS electrical stimuli will be used to obtain hodological information. The results will guide a more extensive unit analysis in the awake squirrel monkey trained to depress a shaft for food reward. A variable intensity light within the shaft will provide intermittent noxious thermal stimuli, producing escape responses with which unit activity will be correlated by means of polygraphic, magnetic and T.V. tape recording. Unit responses will be statistically defined and analyzed. Since there is experimental evidence that an apparently selective analgesia can be produced by electrical stimulation of selected CNS sites, CNS stimulation will be used (1) for hodological information and (2) to further test neural-behavioral correlations during behaviorally identified analgesia.